# vpaintegral

Numerical integration using variable precision

## Syntax

``vpaintegral(f,a,b)``
``vpaintegral(f,x,a,b)``
``vpaintegral(___,Name,Value)``

## Description

example

````vpaintegral(f,a,b)` numerically approximates `f` from `a` to `b`. The default variable `x` in `f` is found by `symvar`. `vpaintegral(f,[a b])` is equal to `vpaintegral(f,a,b)`.```

example

````vpaintegral(f,x,a,b)` performs numerical integration using the integration variable `x`.```

example

````vpaintegral(___,Name,Value)` uses additional options specified by one or more `Name,Value` pair arguments.```

## Examples

### Numerically Integrate Symbolic Expression

Numerically integrate the symbolic expression `x^2` from `1` to `2`.

```syms x vpaintegral(x^2, 1, 2)```
```ans = 2.33333```

### Numerically Integrate Symbolic Function

Numerically integrate the symbolic function y(x) = x2 from `1` to `2`.

```syms y(x) y(x) = x^2; vpaintegral(y, 1, 2)```
```ans = 2.33333```

### High-Precision Numerical Integration

`vpaintegral` uses variable-precision arithmetic while the MATLAB® `integral` function uses double-precision arithmetic. Using the default values of tolerance, `vpaintegral` can handle values that cause the MATLAB `integral` function to overflow or underflow.

Integrate `besseli(5,25*u).*exp(-u*25)` by using both `integral` and `vpaintegral`. The `integral` function returns `NaN` and issues a warning while `vpaintegral` returns the correct result.

```syms u x f = besseli(5,25*x).*exp(-x*25); fun = @(u)besseli(5,25*u).*exp(-u*25); usingIntegral = integral(fun, 0, 30) usingVpaintegral = vpaintegral(f, 0, 30) ```
```Warning: Infinite or Not-a-Number value encountered. usingIntegral = NaN usingVpaintegral = 0.688424 ```

### Increase Precision Using Tolerances

The `digits` function does not affect `vpaintegral`. Instead, increase the precision of `vpainteral` by decreasing the integration tolerances. Conversely, increase the speed of numerical integration by increasing the tolerances. Control the tolerance used by `vpaintegral` by changing the relative tolerance `RelTol` and absolute tolerance `AbsTol`, which affect the integration through the condition

Numerically integrate `besselj(0,x)` from `0` to `pi`, to 32 significant figures by setting `RelTol` to `10^(-32)`. Turn off `AbsTol` by setting it to `0`.

```syms x vpaintegral(besselj(0,x), [0 pi], 'RelTol', 1e-32, 'AbsTol', 0) ```
```ans = 1.3475263146739901712314731279612```

Using lower tolerance values increases precision at the cost of speed.

### Complex Path Integration Using Waypoints

Integrate `1/(2*z-1)` over the triangular path from `0` to `1+1i` to `1-1i` back to `0` by specifying waypoints.

```syms z vpaintegral(1/(2*z-1), [0 0], 'Waypoints', [1+1i 1-1i])```
```ans = - 8.67362e-19 - 3.14159i```

Reversing the direction of the integral, by changing the order of the waypoints and exchanging the limits, changes the sign of the result.

### Multiple Integrals

Perform multiple integration by nesting calls to `vpaintegral`. Integrate

`$\underset{-1}{\overset{2}{\int }}\underset{1}{\overset{3}{\int }}xy\text{\hspace{0.17em}}dx\text{\hspace{0.17em}}dy.$`

```syms x y vpaintegral(vpaintegral(x*y, x, [1 3]), y, [-1 2])```
```ans = 6.0```

The limits of integration can be symbolic expressions or functions. Integrate over the triangular region 0 ≤ x ≤ 1 and |y| < x by specifying the limits of the integration over `y` in terms of `x`.

`vpaintegral(vpaintegral(sin(x-y)/(x-y), y, [-x x]), x, [0 1])`
```ans = 0.89734```

## Input Arguments

collapse all

Expression or function to integrate, specified as a symbolic number, variable, vector, matrix, multidimensional array, function, or expression.

Limits of integration, specified as a list of two numbers, symbolic numbers, symbolic variables, symbolic functions, or symbolic expressions.

Integration variable, specified as a symbolic variable. If `x` is not specified, the integration variable is found by `symvar`.

### Name-Value Pair Arguments

Specify optional comma-separated pairs of `Name,Value` arguments. `Name` is the argument name and `Value` is the corresponding value. `Name` must appear inside quotes. You can specify several name and value pair arguments in any order as `Name1,Value1,...,NameN,ValueN`.

Example: `'RelTol',1e-20`

Relative error tolerance, specified as a positive real number. The default is `1e-6`. The `RelTol` argument determines the accuracy of the integration only if $RelTol·|Q|>AbsTol$, where Q is the calculated integral. In this case, `vpaintegral` satisfies the condition $|Q-I|\le RelTol·|Q|$, where I is the exact integral value. To use only `RelTol` and turn off `AbsTol`, set `AbsTol` to `0`.

Example: `1e-8`

Absolute error tolerance, specified as a non-negative real number. The default is `1e-10`. `AbsTol` determines the accuracy of the integration if $AbsTol>RelTol·|Q|$, where Q is the calculated integral. In this case, `vpaintegral` satisfies the condition $|Q-I|\le AbsTol$, where I is the exact integral value. To turn off `AbsTol` and use only `RelTol`, set `AbsTol` to `0`.

Example: `1e-12`

Integration path, specified as a vector of numbers, or as a vector of symbolic numbers, expressions, or functions. `vpaintegral` integrates along the sequence of straight-line paths (lower limit to the first waypoint, from the first to the second waypoint, and so on) and finally from the last waypoint to the upper limit. For contour integrals, set equal lower and upper limits and define the contour using waypoints.

Maximum evaluations of input, specified as a positive integer or a positive symbolic integer. The default value is `10^5`. If the number of evaluations of `f` is greater than `MaxFunctionCalls`, then `vpaintegral` throws an error. For unlimited evaluations, set `MaxFunctionCalls` to `Inf`.

## Tips

• Ensure that the input is integrable. If the input is not integrable, the output of `vpaintegral` is unpredictable.

• The `digits` function does not affect `vpaintegral`. To increase precision, use the `RelTol` and `AbsTol` arguments instead.